FIG. 3 shows in accordance with the prior art a scroll compressor 10 know which comprises a fixed scroll 12 supported by a compressor casing 14 and an orbiting scroll 16 disposed opposite the fixed scroll supported by a crank shaft 18. The orbiting scroll and the fixed scroll wall form a scroll assembly 19. A motor or drive unit 20 is supplied for causing an orbiting movement of the orbiting scroll 16 in relation to the fixed scroll 12. The fixed scroll comprises a base plate 22 from which a scroll wall 24 extends generally orthogonally. The orbiting scroll 16 comprises a base plate 26 from which an orbiting scroll wall 28 extends generally orthogonally so as to co-operate with the fixed scroll wall 28 for compressing fluid between inlet 30 and an outlet 32 of the compressor 10 when the orbiting scroll 16 orbits about the fixed scroll 12.
In the arrangement shown, the orbiting scroll 16 is disposed towards the centre of the compressor 10 (i.e. towards the left of the fixed scroll in FIG. 3) and towards the moving parts of the compressor. This causes the orbiting scroll 16 to increase in temperature during use and causes thermal expansion of the orbiting scroll. The orbiting scroll 16 does not easily dissipate heat because generally it is positioned at a low pressure side of the compressor where conductive heat transfer to the fluid being pumped is limited and there is no access for ambient air. The fixed scroll 12 on the other hand is positioned with its rear face in the ambient air which is used to provide cooling. It will be understood therefore that the orbiting scroll 16 undergoes thermal expansion relative to the fixed scroll 12 between ambient temperature when the compressor is not in use (or when the components of the compressor are at the same temperature) and working temperatures when the pump is in use. A converse arrangement is possible, but not shown, in which the relative orientation of the fixed scroll and the orbiting scroll leads to the fixed scroll being heated and expanding relative to the orbiting scroll, although this arrangement is not currently preferred and will not be discussed further.
Thermal expansion of the orbiting scroll 16 translates into radial expansion of base plate 26. The radial expansion is generally dependent on the distance from the centre of the plate so that a radially outer portion of the plate expands more than a radially inner portion of the plate and accordingly, a radially outer portion of the orbiting scroll wall 28 expands more than a radially inner portion thereof. The inner section of the scroll wall expands about 10 to 50 (typically 30) microns whilst radially outer portions may expand by many times the inner clearance, about 100 to 500 microns (typically 200 to 300 microns for a 50° C. rise depending on the diameter of the scroll).
FIGS. 4 to 8 show cross-sections of the prior art scroll assembly, taken along line II in FIG. 3. Fluid enters the scroll arrangement at inlet 30 where orbiting movement of the orbiting scroll causes it to be compressed along two fluid flow paths and to be exhausted from outlet 32. The first path passes between a first two facing wall surfaces, that is, a radially outer wall surface 36 of the fixed scroll wall 28 and a radially inner wall surface 38 of the orbiting scroll wall 24. The second path extends between a second two facing surfaces, that is, a radially inner wall surface 40 of the fixed scroll wall 28 and a radially outer wall surface 42 of the orbiting scroll wall 24. Fluid on the first flow path is trapped in crescent-shaped fluid pockets 44 which are forced to shrink in size as they are caused to move inwardly by the motion of the orbiting scroll wall as can be seen from a comparison of the position of the single highlighted pocket 44 shown in FIGS. 4 to 8. Although a single pocket 44 is highlighted in FIGS. 4 to 8, it will be seen that the first fluid flow path contains as many pockets of trapped fluid being compressed as there are wraps of the scroll walls. In the same way, fluid on the second fluid flow path is trapped in crescent-shaped fluid pockets 46 and is forced inwardly by motion of the orbiting scroll 16.
During compression, each fluid pocket 44,46 extends for less than 3608 about the circumference of the scroll assembly. The first two wall surfaces 36,38 are separated by just enough space, or clearance, at the circumferential ends of the pockets 44 to resist the seepage of fluid. The second two facing wall surfaces 40,42 are also separated by a clearance at each circumferential end of pockets 46. These clearances are hereinafter referred to as running, or working, clearances. No sealant or lubricant is, therefore, required in the swept volume of the pump.
As will be seen from FIG. 4, fluid pockets on the first fluid path extend between clearances C1 and fluid pockets on the second fluid path extend between clearances C2. Clearances C1 are substantially radially aligned and clearances C2 are substantially radially aligned, however, clearances C1 and C2 are substantially diametrically opposed in the scroll assembly.
It is important to accurately maintain running clearances between the scroll walls since if the running clearance is too large seepage out of the pockets occurs leading to loss in efficiency. If the running clearance is too small, there is a possibility that the scroll walls collide. It is apparent that thermal expansion of one of the scroll walls affects the running clearances between the scroll walls between ambient and running conditions. This thermal expansion causes a problem which will be explained with reference to expansion of the orbiting scroll wall 28 relative to the fixed scroll wall 24. First, the radially outer wall surface 36 of the orbiting scroll wall 28 expands towards the radially inner wall surface 38 of the fixed scroll wall 24 thereby reducing clearance C1 with the risk of collision between the scroll walls. Secondly, the radially inner wall surface 40 of the orbiting scroll wall 28 expands away from the radially outer wall surface 42 of the fixed scroll 24 thereby increasing the clearance C2 therebetween and causing seepage. It is desirable therefore that when the pump is at ambient temperature (i.e. all components are at the same temperature), the scrolls do not collide with each other, but when the pump is at running temperature the clearances are neither too small that the scrolls collide nor too large that the pump does not achieve its vacuum performance.
FIG. 9 shows a representation of the relationship between running clearances C1 and C2 and clearances A1 and A2 (where ‘A1’ represents the clearance at ambient temperatures between the first two facing wall surfaces 36 and 38, and ‘A2’ represents the clearance at ambient temperatures between the second two facing wall surfaces 40 and 42). The relationship is plotted between the exhaust (radial centre) and the inlet (outer radial portion) of the scroll assembly. It will be seen that FIG. 9 does not show the actual spacing between the orbiting scroll wall 28 and the fixed scroll wall 24, which would be represented by cyclic curves forming pockets 44,46.
According to the prior art, sufficient ambient clearance A1 is provided between the first two wall surfaces 36 and 38 to allow the orbiting scroll wall to expand without colliding with the fixed scroll wall and so that at working conditions a desired running clearance C1 is achieved. According to the prior art, the ambient clearance A1 is increased by angularly displacing the orbiting scroll wall relative to the fixed scroll wall. This angular displacement causes the radius of the orbiting scroll wall to be reduced relative to the fixed scroll wall at any given angle about the centre of the scroll assembly, even though the actual shape and pitch of both scroll walls remains the same. If ambient clearances A1 are increased by this angular displacement, ambient clearance A2 will be decreased. As shown in FIG. 9, clearance A1 is the same as clearance A2. At running temperatures, running clearance C1 gradually reduces towards the inlet of the scroll assembly since thermal expansion increases depending on the radial distance from the centre of the scroll assembly. Conversely, running clearance C2 gradually increases towards the inlet of the scroll assembly. As shown, the orbiting scroll wall 28 collides with the fixed scroll wall 24 towards the inlet of the scroll assembly. Further, compression of fluid on the first and the second fluid flow paths are different because C1 is less than C2 and therefore more seepage occurs in the second flow path thereby reducing efficiency.
A second prior art scroll compressor is described with reference to FIG. 10 which shows the same relationship between ambient clearances A1 and A2, and running clearances C1 and C2 as shown in FIG. 9. The second prior art scroll compressor to some extent reduces the extent of the problem highlighted above.
In the second depicted prior art scroll compressor, ambient clearance A1 between a first two facing wall surfaces 50,52 gradually increases as the radial distance from the centre of the scroll assembly increases and ambient clearance A2 between a second two facing wall surfaces 54,56 gradually decreases as the radial distance from the centre of the scroll assembly increases such that the rate of change of A1 and A2 are equal and respectively constant. The first two facing wall surfaces 50,52 are, respectively, a radially inner surface 50 of a fixed scroll wall 58 and a radially outer surface of an orbiting scroll wall 60. The second two facing wall surfaces 54,56 are, respectively, a radially inner surface 50 of the orbiting scroll wall 60 and a radially outer surface of the fixed scroll wall 58.
The above relationship between A1 and A2 is enabled by providing the orbiting scroll wall 60 with a spiral with a different pitch to that of the fixed scroll wall 58. In more detail, the orbiting scroll wall 60 has a spiral with reduced pitch in that its radius increases more slowly as it extends away from its centre than the increase in radius of the fixed scroll wall 58. Therefore, as the orbiting scroll wall 60 extends radially outwardly, A1 gradually increases to compensate for the affect of thermal expansion which increases as distance from the centre (exhaust) increases. As will be seen in FIG. 10, A1 is increased as compared to the prior art in FIG. 9. The second prior art scroll compressor allows for greater thermal expansion of the orbiting scroll wall without colliding with the fixed scroll wall at running temperatures, and without allowing C2 to increase to allow significant seepage of gas between the second two facing wall surfaces 54,56. However, clearance C1 and C2 are not equal and therefore there will be some difference between fluid compression on the first fluid path and on the second fluid path. However, ambient clearance A2, particularly towards the inlet, cannot be further increased without the risk of collision between the scroll walls.
It is desirable to provide an improved solution to the above problem.